The present invention relates to a process for the production of acetic acid and in particular, to a process for the production of acetic acid by carbonylation in the presence of an iridium catalyst and methyl iodide co-catalyst.
Preparation of carboxylic acids by iridium-catalysed carbonylation processes is known and is described, for example in EP-A-0786447, EP-A0643034 and EP-A-0752406.
EP-A-0786447 describes a process for reacting carbon monoxide with a carbonylatable reactant and/or an ester derivative thereof in a liquid reaction composition comprising an iridium carbonylation catalyst, a hydrocarbyl halide, water and carbonylation reaction product, characterised in that the liquid reaction composition comprises water at a concentration of 2 to 8% by weight, hydrocarbyl halide at a concentration in the range 1 to 20% by weight and ester derivative of the carbonylatable reactant at a concentration in the range 1.0 to 60% by weight.
EP-A-0643034 describes a process for the production of acetic acid by carbonylation of methanol or a reactive derivative thereof which process comprises contacting methanol or a reactive derivative thereof with carbon monoxide in a liquid reaction composition in a carbonylation reactor characterised in that the liquid composition comprises (a) acetic acid, (b) an iridium catalyst, (c) methyl iodide, (d) at least a finite quantity of water, (e) methyl acetate and (f) as promoter, at least one of ruthenium and osmium.
EP-A-0752406 describes a process for the production of acetic acid comprising (1) continuously feeding methanol and/or a reactive derivative thereof and carbon monoxide to a carbonylation reactor which contains a liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide co-catalyst, a finite concentration of water, acetic acid, methyl acetate and at least one promoter; (2) contacting the methanol and/or reactive derivative thereof with the carbon monoxide in the liquid reaction composition to produce acetic acid; and (3) recovering acetic acid from the liquid reaction composition characterised in that there is continuously maintained in the liquid reaction composition throughout the course of the reaction (a) water at a concentration of no greater than 6.5% by weight, (b) methyl acetate at a concentration in the range 1 to 35% by weight and (c) methyl iodide at a concentration in the range 4 to 20% by weight.
The use of polydentate chelating phosphorus or arsenic ligands in carbonylation processes is known, for example from U.S. Pat. No. 4,102,920 and U.S. Pat. No. 4,102,921 which describe respectively, their use in rhodium and iridium catalysed carbonylation processes.
The use of phosphine oxide promoters in rhodium-catalysed carbonylation processes is known from U.S. Pat. No. 5,817,869 and from EP-A-01 14703.
Thus, U.S. Pat. No. 5,817,869 relates to process for the production of acetic acid without the use of an alkali metal halide comprising contacting methanol or methyl acetate with carbon monoxide in the presence of a carbonylation system containing about 200 to about 1200 ppm of rhodium-containing component and a liquid reaction medium comprising about 20 to about 80 weight % acetic acid; from about 0.6 to about 36 weight % methyl iodide; from about 0.5 to about 10 weight % methyl acetate, said contacting being in the presence of at least one pentavalent Group VA oxide of the formula R3M=O, which is present in a concentration of Group VA oxide to rhodium of greater than about 60:1, and water being added in an amount of from about 4 to about 12 weight %.
EP-A-0114703 relates to a process for the preparation of carboxylic acids and/or esters by reaction of an alcohol with carbon monoxide in the presence of a rhodium compound, an iodide and/or bromide source and a phosphorus, arsenic or antimony-containing compound as promoter, characterised in that the reaction is carried out in the presence of a compound of the formula 
wherein X represents phosphorus, arsenic or antimony and Y oxygen, sulphur or selenium and either a and b, independent of one another, are 0 or 1, R1 represents hydrogen or an unsubstituted or substituted hydrocarbon group and R2 and R3 each represent an unsubstituted or substituted hydrocarbon group, or a and b are 0 and R2 and R3 together with X form a heterocyclic group and R1 represents hydrogen or an unsubstituted or substituted hydrocarbon group, or in the presence of a complex of a compound of formula I with a hydrocarbon iodide or bromide, an acyl iodide or bromide or hydrogen iodide or bromide. Examples of compounds of formula I given, include 
According to EP-A-0114703, the quantity of compound of formula I used as promoter in the process may vary within wide limits, for instance between 0.1 and 300 mol per gram atom rhodium. Preference is said to be given to use of 1-200, in particular 10-100 mol per gram atom rhodium. The promoters of EP-A-0114703 are directed towards improving the activity of the rhodium carbonylation catalyst system.
The technical problem to be solved is to provide an improved carbonylation process for the production of acetic acid. It has now been surprisingly found that by using a small quantity of a polydentate phosphine oxide compound in an iridium-catalysed carbonylation process for the production of acetic acid, the quantities of by-product propionic acid, its precursors and derivatives produced are reduced and hence selectivity to the desired acetic acid is increased.
Thus, according to the present invention, there is provided a process for the production of acetic acid by reacting carbon monoxide with methanol and/or a reactive derivative thereof in a liquid reaction composition comprising an iridium carbonylation catalyst, methyl iodide, methyl acetate, water and acetic acid characterised in that there is also present in the reaction composition a polydentate phosphine oxide compound in an amount of less than 10 mol per gram atom of iridium.
The process of the present invention solves the technical problem defined above, by the use of a polydentate phosphine oxide compound in an amount of less than 10 mol per gram atom of iridium to reduce the amount of by-product propionic acid, its precursors such as ethyl iodide and ethyl acetate and its derivatives such as methyl propionate and ethyl propionate produced and increase the selectivity of the process to the desired acetic acid.
The polydentate phosphine oxide compound may be represented by the formula: 
wherein Q1, Q2, Q3 and Q4 are independently C1 to C10 alkyl, C6 to C15 aryl groups, C1 to C10 alkoxy or C6 to C15 aryloxy groups, optionally substituted with substituents selected from the group consisting of xe2x80x94NO3, xe2x80x94OH, xe2x80x94CN, xe2x80x94SO3H, xe2x80x94OCH3 and xe2x80x94CO2H, and Z is a divalent saturated or unsaturated hydrocarbyl group, preferably xe2x80x94(CH2)xxe2x80x94wherein x is an integer from 1 to 6 inclusive, preferably from 1 to 3 inclusive; the Z group may optionally be substituted with C1 to C10 alkyl, C5 to C15 aryl, C1 to C10 alkoxy, C5 to C15 aryloxy groups or OH groups. Where Z is an unsaturated hydrocarbyl group it may be, for example, C6H4 that is 
which may be optionally substituted with substituents selected from the group consisting of xe2x80x94NO3, xe2x80x94OH, xe2x80x94CN, xe2x80x94SO3H, xe2x80x94OCH3, xe2x80x94CO2H, C1 to C10 alkyl, C5to C15 aryl, C1 to C10 alkoxy and C5to C15 aryloxy groups. Suitable polydentate phosphine oxide compounds of formula II are:
(Ph)2P(O)xe2x80x94CH2xe2x80x94P(O)(Ph)2xe2x80x83xe2x80x83IIa,
(Ph)2P(O)xe2x80x94(CH2)2xe2x80x94P(O)(Ph)2xe2x80x83xe2x80x83IIb and
(Ph)2P(O)xe2x80x94(CH2)3xe2x80x94P(O)(Ph)2xe2x80x83xe2x80x83IIc
wherein Ph represents a phenyl group.
The polydentate phosphine oxide compound may also be represented by the formula: 
wherein Q5, Q6, Q7 and Q8 are independently C1 to C10 alkyl, C6 to C15 aryl groups, C1 to C10 alkoxy or C6to C15 aryloxy groups, optionally substituted with substituents selected from the group consisting of xe2x80x94NO3, xe2x80x94OH, xe2x80x94CN, xe2x80x94SO3H, xe2x80x94OCH3 and xe2x80x94CO2H, and Zxe2x80x2 and Zxe2x80x3 are independently divalent saturated or unsaturated hydrocarbyl groups, preferably xe2x80x94(CH2)xxe2x80x2xe2x80x94and xe2x80x94(CH2)xxe2x80x3xe2x80x94respectively wherein xxe2x80x2 and xxe2x80x3 are independently integers from 1 to 6 inclusive, preferably from 1 to 2 inclusive; the Zxe2x80x2 and Zxe2x80x3 groups may optionally be independently substituted with C1 to C10 alkyl, C15 to C15 aryl, C1 to C10 alkoxy, C5 to C15 aryloxy groups or OH groups. Where Zxe2x80x2 or Zxe2x80x3 is an unsaturated hydrocarbyl group it may be, for example, C6H4 that is 
which may be optionally substituted with substituents selected from the group consisting of xe2x80x94NO3, xe2x80x94OH, xe2x80x94CN, xe2x80x94SO3H, xe2x80x94OCH3, xe2x80x94CO2H, C1 to C10 alkyl, C5 to C15 aryl, C1 to C10 alkoxy and C5 to C15 aryloxy groups.
The polydentate phosphine oxide compound may also be represented by the formula: 
wherein Q9, Q10, Q11, Q12, Q13 and Q14 are independently C1 to C10 alkyl, C6to C15 aryl groups, C1 to C10 alkoxy or C6 to C15 aryloxy groups, optionally substituted with substituents selected from the group consisting of xe2x80x94NO3, xe2x80x94OH, xe2x80x94CN, xe2x80x94SO3H, xe2x80x94OCH3 and xe2x80x94CO2H, and Zxe2x80x2xe2x80x3 is a trivalent saturated hydrocarbyl groups, for example (xe2x80x94CH2)3CCH3.
The polydentate phosphine oxide compound may be present in the reaction composition in an amount of greater than 0.5 mol per gram atom of iridium. Preferably, the polydentate phosphine oxide compound is present in the reaction composition in an amount of at least 1 mol per gram atom of iridium and less than 10 mol per gram atom of iridium.
In the process of the present invention, the iridium carbonylation catalyst is preferably present in the liquid reaction composition at a concentration in the range 400 to 5000 ppm measured as iridium, more preferably in the range 500 to 3000 ppm measured as iridium. Any reduction in carbonylation rate caused by the presence of the polydentate phosphine oxide compound may be off-set by increasing the concentration of iridium catalyst.
The iridium catalyst in the liquid reaction composition may comprise any iridium containing compound which is soluble in the liquid reaction composition. The iridium catalyst may be added to the liquid reaction composition for the carbonylation reaction in any suitable form which dissolves in the liquid reaction composition or is convertible to a soluble form.
In the process of the present invention, the concentration of methyl iodide co-catalyst in the liquid reaction composition is preferably in the range 5 to 16% by weight.
In the process of the present invention, suitable reactive derivatives of methanol include methyl acetate, dimethyl ether and methyl iodide. A mixture of methanol and reactive derivatives thereof may be used as reactants in the process of the present invention. Preferably, methanol and/or methyl acetate are used as reactants. At least some of the methanol and/or reactive derivative thereof will be converted to, and hence present as, methyl acetate in the liquid reaction composition by reaction with acetic acid product or solvent. In the process of the present invention the concentration of methyl acetate in the liquid reaction composition is preferably in the range 1 to 30% by weight, more preferably 5 to 25% by weight.
Water may be formed in situ in the liquid reaction composition, for example, by the esterification reaction between methanol reactant and acetic acid product. Small amounts of water may also be produced by hydrogenation of methanol to produce methane and water. Water may be introduced to the carbonylation reactor together with or separately from other components of the liquid reaction composition. Water may be separated from other components of reaction composition withdrawn from the reactor and may be recycled in controlled amounts to maintain the required concentration of water in the liquid reaction composition. The water concentration in the liquid reaction composition is suitably in the range 1-15 wt %, preferably in the range 1-6.5 wt %.
Preferably, in the process of the present invention at least one promoter is present in the reaction composition. In the process of the present invention it has been found that the beneficial effect of the polydentate phosphine oxide is greater in the presence of a promoter. Suitable promoters are preferably selected from the group consisting of ruthenium, osmium, rhenium, cadmium, mercury, zinc, gallium, indium and tungsten, and are more preferably selected from the group consisting of ruthenium and osmium and most preferably is ruthenium. Preferably, the promoter is present in an effective amount up to the limit of its solubility in the liquid reaction composition and/or any liquid process streams recycled to the carbonylation reactor from the acetic acid recovery stage. The promoter is suitably present in the liquid reaction composition at a molar ratio of promoter: iridium in the range 0.5:1 to 15:1. Any reduction in carbonylation rate caused by the presence of the polydentate phosphine oxide compound may be off-set by increasing the concentration of promoter, if present.
The promoter may comprise any suitable promoter metal-containing compound which is soluble in the liquid reaction composition. The promoter may be added to the liquid reaction composition for the carbonylation reaction in any suitable form which dissolves in the liquid reaction composition or is convertible to soluble form.
Preferably, the iridium- and promoter-containing compounds are free of impurities which provide or generate in situ ionic iodides which may inhibit the reaction, for example, alkali or alkaline earth metal or other metal salts.
Ionic contaminants such as, for example, (a) corrosion metals, particularly nickel, iron and chromium and (b) phosphines or nitrogen containing compounds or ligands which may quaternise in situ; should be kept to a minimum in the liquid reaction composition as these will have an adverse effect on the reaction by generating Ixe2x88x92 in the liquid reaction composition which has an adverse effect on the reaction rate. Some corrosion metal contaminants such as for example, molybdenum have been found to be less susceptible to the generation of Ixe2x88x92. Corrosion metals which have an adverse affect on the reaction rate may be minimised by using suitable corrosion resistant materials of construction. Similarly, contaminants such as alkali metal iodides, for example lithium iodide, should be kept to a minimum. Corrosion metal and other ionic impurities may be reduced by the use of a suitable ion exchange resin bed to treat the reaction composition, or preferably a catalyst recycle stream. Such a process is described in U.S. Pat. No. 4007130. Preferably, ionic contaminants are kept below a concentration at which they would generate 500 ppm Ixe2x88x92, preferably less than 250 ppm Ixe2x88x92 in the liquid reaction composition.
The carbon monoxide reactant may be essentially pure or may contain inert impurities such as carbon dioxide, methane, nitrogen, noble gases, water and C1 to C4 paraffinic hydrocarbons. The presence of hydrogen in the carbon monoxide feed and generated in situ by the water gas shift reaction is preferably kept low as its presence may result in the formation of hydrogenation products. Thus, the amount of hydrogen in the carbon monoxide reactant is preferably less than 1 mol %, more preferably less than 0.5 mol % and yet more preferably less than 0.3 mol % and/or the partial pressure of hydrogen in the carbonylation reactor is preferably less than 1xc3x97105 N/m2 partial pressure, more preferably less than 5xc3x97104 N/m2 and yet more preferably less than 3xc3x97104 N/m2. The partial pressure of carbon monoxide in the reactor is suitably in the range 1xc3x97105 N/m2 to 7xc3x97106 N/m2, preferably 1xc3x97105 N/m2 to 3.5xc3x97106 N/m2, more preferably 1xc3x97105 N/m2 to 1.5xc3x97106 N/m2.
The total pressure of the carbonylation reaction is suitably in the range 1xc3x97106 N/m2 to 2xc3x97107 N/m2, preferably 1.5xc3x97106 N/m2 to 1xc3x97107 N/m2, more preferably 1.5xc3x97106 N/m2 to 5xc3x97106 N/m2.
The temperature of the carbonylation reaction is suitably in the range 100 to 300xc2x0 C., preferably in the range 150 to 220xc2x0 C. Any reduction in carbonylation rate caused by the presence of the polydentate phosphine oxide compound may be off-set by increasing the reaction temperature.
The process of the present invention is preferably performed as a continuous process.
The acetic acid product may be recovered from the liquid reaction composition by withdrawing vapour and/or liquid from the carbonylation reactor and recovering acetic acid from the withdrawn material. Preferably, acetic acid is recovered from the liquid reaction composition by continuously withdrawing liquid reaction composition from the carbonylation reactor and recovering acetic acid from the withdrawn liquid reaction composition by one or more flash and/or fractional distillation stages in which the acetic acid is separated from the other components of the liquid reaction composition such as iridium catalyst, methyl iodide co-catalyst, promoter, methyl acetate, unreacted methanol, water and acetic acid solvent which may be recycled to the reactor to maintain their concentrations in the liquid reaction composition. To maintain stability of the iridium catalyst during the acetic acid product recovery stage, water in process streams containing iridium carbonylation catalyst for recycle to the carbonylation reactor should be maintained at a concentration of at least 0.5% by weight.
The process of the present invention may be performed a using carbonylation reaction conditions known in the art, for example as described in EP-A-0786447, EP-A-0643034, EP-A-0752406 and EP-A-0749948, the contents of which are hereby incorporated by reference.
The invention will now be illustrated by way of example only and with reference to the following examples:
General Reaction Method
A 300 cm3 zirconium autoclave, equipped with a stirrer and a liquid injection facility, was used for a series of batch autoclave experiments. The autoclave was pressure tested to 4xc3x97106 N/m2 with nitrogen and then flushed three times with carbon monoxide up to 1xc3x97106 N/m2. An initial charge consisting of a methyl ester (usually methyl acetate (approx. 48.0 g)), acetic acid (approx. 70.0 g), methyl iodide (approx. 8.9 g) and water (approx. 10.0 g), was placed into the autoclave, which was then re-purged with carbon monoxide to 4xc3x97106 N/M2 and vented slowly so as not to lose any volatiles. Then carbon monoxide (approx. 4-5xc3x97105 N/m2) was placed in the autoclave which was then heated with stirring (1500 rpm) to 190xc2x0 C. A catalyst solution was primed into a liquid injection line with (approx. 1.35 g) of H2IrCl6 solution (22.26% Ir w/w), water (approx. 6.0 g) and acetic acid (approx. 6.0 g) and injected with an over-pressure of carbon monoxide to the hot autoclave to bring the autoclave pressure to 2.8xc3x97106 N/m2.
The reaction rate was monitored by drop in carbon monoxide pressure from a ballast vessel, typically charged to 7xc3x97106 N/m2. The autoclave pressure and temperature were maintained at a constant 2.8xc3x97106 N/m2 and 190xc2x0 C. throughout the reaction by pressure and coolant control valves. The reaction was terminated when the drop in ballast pressure became less than 1xc3x97104 N/m2 per 5 minutes.
After cooling and carefully venting the autoclave the liquid components were discharged and analysed for liquid products and by-products by known established gas chromatography methods.
Liquid by-products are determined by gas chromatography using a CB wax52 column on a Hewlett Packard 6820 Mk2 gas chromatograph. Detected components are quantified by integration of the component peaks relative to an external standard and expressed as parts per million (ppm) by weight.
The main liquid by-product from carbonylation of methanol to acetic acid is propionic acid. Precursors (ethyl iodide and ethyl acetate) are also formed. In a continuous process these precursors would be recycled to the carbonylation reactor in recycle streams, building up to a steady state concentration at which the rate of their destruction to propionic acid balances their rate of removal. In a batch process, these precursors are not destroyed, but accumulate with the propionic acid in the liquid reaction composition and these can be measured at the end of the experiment. A reduction in the amount of propionic acid and its precursors measured at the end of a batch carbonylation experiment would be expected to indicate that in a continuous process, the amount of by-product propionic acid recovered with the acetic acid product would also be reduced.
In the batch reactions xe2x80x98Totalxe2x80x99 propionic acid was defined as the sum of propionic acid and it""s precursors ((ethyl acetate and ethyl iodide) converted to ppm propionic acid), detected in the quenched liquid products of the batch reaction and expressed in ppm.
Total propionic acid=ppm propionic acid+(ppm ethyl iodidexc3x97(74.08/155.97))+(ppm ethyl acetatexc3x97(74.08/88.11))
This represents the cumulative formation during the batch experiment of propionic acid and its precursors, ethyl iodide and ethyl acetate. Ethanol and acetaldehyde are produced in very small amounts such that they can be ignored.
The rate of gas uptake at a certain point in a reaction run was used to calculate the carbonylation rate, as number of moles of reactant consumed per litre of cold degassed reactor composition per hour (mol/l/hr), at a particular reactor composition (total reactor composition based on a cold degassed volume).
The methyl acetate concentration was calculated during the course of the reaction from the starting composition, assuming that one mole of methyl acetate was consumed for every mole of carbon monoxide that was consumed. No allowance was made for organic components in the autoclave headspace.
By monitoring the rate of carbonylation reaction and calculating the concentration of the reaction components during the experiment, it is possible to determine the rate of carbonylation reaction which would be expected if a carbonylation process were to be operated continuously whilst maintaining under steady state, a liquid reaction composition which is the same as the total reaction composition calculated at any particular point in the batch experiment. In the batch experiments the term reaction composition"" refers to the total composition of the components in the autoclave in the cold degassed state. The principal difference between the batch experiments and continuous operation is that in the batch experiments, no allowance was made in calculating the component concentrations, for partitioning of the reaction components between the liquid and gaseous phases. Owing to this partitioning, the concentration of the reaction components present in the liquid phase in a batch reaction under reaction conditions was similar, but not identical, to the total reaction composition. In particular, the more volatile components in the reaction composition, such as methyl iodide and methyl acetate, were slightly less concentrated in the liquid reaction composition than in the total reaction composition, whereas the water concentration was comparable between the two. Therefore, the rate calculated in a batch experiment at a certain total reaction composition will be similar to that in a continuous process operating with a liquid composition which is the same as the batch total reaction composition. In addition, trends observed in batch experiments by varying the process variables, such as water concentration, were comparable with the trends observed in continuous experiments.
A baseline experiment was performed with the autoclave charged with methyl acetate (48.10 g), acetic acid (68.99 g), water (9.94 g), methyl iodide (8.84 g). The catalyst solution consisted of an iridium solution (46.9% H2IrCl6, 53.1% water, 1.457 g) with acetic acid (6.5 g) and water (6.5 g).
The rate of reaction, based on carbon monoxide uptake, was measured to be 9.4 mol/l/hr at a calculated reaction composition of 12.5% methyl acetate and steadily declined until virtually all the methyl acetate was consumed. Conversion to acetic acid was 86% based on methyl acetate consumed. Analysis of propionic acid precursors gave a total propionic acid make of 260 ppm. Gaseous by-products in the cold-vented off-gas were H2, 4.5 mmol; CO2, 5.1 mmol; CH4, 7.4 mmol.